The company supports its business products for 10 years, the first five in what it calls "mainstream support," and the second five in "extended support." The biggest difference between the two phases is that in the latter, non-security fixes are provided only to companies that have signed support contracts with Microsoft.

With Office XP falling off the support list, Microsoft urged users to upgrade to a newer edition.

Office 2003, which left mainstream support two years ago, is eligible for security updates until Jan. 14, 2014. Office 2007 and Office 2010 will receive patches until April 2017 and October 2020, respectively.

Users can continue to run out-of-support software -- there's no "kill switch" that gets thrown -- but without patches to plug holes, they may be at greater risk to hackers' exploits.

Microsoft last upgraded Office XP in 2004 when it released Service Pack 3 (SP3) for the suite.